Isoflavones are biologically active compounds occurring naturally in a variety of plants, with relatively high levels found in soybeans. Twelve laboratories participated in a collaborative study to determine the aglycon isoflavone content of 8 test samples of soy and foods containing soy. The analytical method for the determination of isoflavones incorporates a mild saponification step that reduces the number of analytes measured and permits quantitation versus commercially available, stable reference standards. Test samples were extracted at 65°C with methanol–water (80 + 20), saponified with dilute sodium hydroxide solution, and analyzed by reversed-phase liquid chromatography with UV detection at 260 nm. Isoflavone results were reported as μg/aglycon/g or μg aglycon equivalents/g. The 8 test samples included 2 blind duplicates and 4 single test samples with total isoflavone concentrations ranging from approximately 50 to 3000 μg/g. Test samples of soy ingredients and products made with soy were distributed to collaborators with appropriate reference standards. Collaborators were asked to analyze test samples in duplicate on 2 separate days. The data were analyzed for individual isoflavone components, subtotals of daidzin–daidzein, glycitin–glycitein, and genistin–genistein, and total isoflavones. The relative standard deviation (RSD) for repeatability was 1.8–7.1%, and the RSD for reproducibility was 3.2–16.1% for total isoflavone values of 47–3099 μg/g.

We have developed a reversed-phase liquid-chromatographic procedure for simultaneously determining phenylalanine and tyrosine in serum and eluates of dried blood spots. Batch derivatization with phenylisothiocyanate and a 10-min linear gradient chromatographic assay with ultraviolet absorbance detection provide rapid sample throughput. Interrun precision (CV) is less than 12%; analytical recovery (from blood spot samples) exceeds 85%. Results for patients' samples correlate well with those from an amino acid analyzer and we encountered no apparent interferences. The speed and specificity of this assay facilitate the rapid diagnosis and monitoring of patients with phenylketonuria.